GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 109 



weight will either lessen the rate at which the curve rises and decrease the 

 height, or, if the weight be not great, it may acquire a velocity from the energy 

 suddenly imparted to it by the muscle, which will carry the record higher than 

 the absolute contraction of the muscle. 



(3) The fall of the curve may be altered. The weight, suddenly freed by the 

 rapidly relaxing muscle, may acquire a velocity in falling which will stretch 

 the muscle-tissue, carry the record lower than the actual relaxation of the 

 muscle would warrant, and lead to the development of artificial elastic after- 

 oscillations. 



These sources of error can be in part overcome by the employment of an 

 exceedingly light, stiff writing-lever, and by bringing the necessary tension on 

 the muscle by placing the extending weight very near the axis of the lever, so 

 that it shall move but little and hence acquire little velocity. 



(c) Effect of Rate of Excitation on Height and Form of Muscular Contrac- 

 tion. If a muscle be excited a number of times by exactly the same irritant 

 and under the same external conditions, the amount and course of each of 

 the contractions should be exactly the same, provided the condition of the 

 muscle itself remains the same. The condition of the muscle is, however, 

 altered every time it is excited to contraction, and each contraction leaves 

 behind it an after-effect. This altered condition is not permanent ; as we have 

 seen, increased katabolisnx is accompanied by increased anabolism, and, if the 

 excitations do not follow each other too rapidly, the katabolic changes occur- 

 ring in contraction are compensated for by anabolic changes during the suc- 

 ceeding interval of rest. Normally, a muscle, under the restorative influence 

 of the blood, rapidly recovers from the alterations produced by the contraction 

 process, and, therefore, if not excited too frequently, will give, other things 

 being equal, the same response each time it is called into action. The best 

 illustration of this is the heart, which continues to beat at a regular rate 

 throughout the life of the individual. Tiegel found that one of the skeletal 

 muscles of a frog, while in the normal body, can make more than a thousand 

 contractions in response to artificial stimuli without showing fatigue; finally 

 the effect of the work shows itself in a lessening of the power to contract. 

 Every muscle contains a surplus of energy-holding compounds and also sub- 

 stances capable of neutralizing waste products, and even a muscle which has 

 been separated from the rest of the body retains for a considerable time the 

 ability to recover from the effects of excitation. It is evident that when a 

 muscle is excited repeatedly, a certain interval of rest must be permitted 

 between the succeeding excitations if its normal condition is to be maintained, 

 and that the more extensive the chemical changes produced by the excita- 

 tions the longer must be the periods allowed for recovery. This being the 

 case, the rate of excitation and consequent length of the interval of rest will 

 have a great effect upon the condition of the muscle and its capacity for work. 



(1) Effect of Frequent Excitations on the Height of Separate Muscular 

 Contractions. Other things being equal, the height to which a muscle can con- 

 tract when excited by a given irritant can be taken as an index of its capacity 



